For its thermochromic properties, vanadium dioxide has been proposed for use as, for example, an automatic dimming material that automatically blocks infrared rays (heat rays) at high temperatures in summer while transmitting them at low temperatures in winter. Such an automatic dimming material, when applied to windows of automobiles or buildings, is expected to improve air conditioning efficiency by automatically adjusting the temperature inside the automobiles or the rooms. The automatic dimming material may be in the form of a thin film or film. For production of thin-film automatic dimming materials, dry film forming methods, such as sputtering, have been studied. However, as these methods require high cost and nave difficulty in forming a film having a large area, production methods including application or printing using fine particles have been proposed. For example, Patent Literature 1 discloses a method of forming a vanadium dioxide-dispersed resin layer by applying, on an appropriate substrate, a composition containing vanadium dioxide fine particles, a light-transmitting resin, and an organic solvent capable of dissolving the light-transmitting resin. A film of an automatic dimming material can be produced by, for example, dispersing vanadium dioxide fine particles in a resin, kneading them, and pressing or extrusion molding the kneaded product A laminated glass including two glass plates and the above film interposed therebetween can also be produced.
Patent Literature 2 discloses an interlayer film for laminated glass containing vanadium dioxide particles, and, the method for producing the same. Such an interlayer film for laminated glass containing vanadium dioxide fine particles dispersed therein is expected to highly transmit infrared rays below the phase transition temperature of vanadium dioxide and to block infrared rays above the phase transition temperature. However, such an inters ayes film for laminated glass containing vanadium dioxide particles dispersed therein shows a decrease in the thermochromic properties with time during storage or use, and thus has low durability.
Improvement in the properties of vanadium dioxide particles themselves has also been attempted to improve the thermochromic properties. It is known that vanadium dioxide particles with a smaller particle size have lower crystallinity. The thermochromic properties of vanadium dioxide particles greatly depend on their crystallinity, and usually vanadium dioxide particles with higher crystallinity tend to have better thermochromic properties. For improved transparency, nanoparticles with a particle size of 100 nm or smaller are required. Such a reduced particle however, leads to significantly low thermochromic properties.
In order to overcome this, vanadium dioxide nanoparticles are fired at a high temperature to provide particles with high crystallinity. However, this method produces coarse particles composed of sintered particles, and fails to produce vanadium dioxide nanoparticles having a small particle size.